1. Field of the Invention
This invention generally relates to an electronic countermeasure system which retransmits a phase shifted form of the illuminating victim radar signal thereby causing return wave front distortion.
2. Description of the Prior Art
The VanAtta array principle is known and involves the retransmission of an illuminating radar pulse back toward its source. The xe2x80x9ccross-eyexe2x80x9d countermeasure technique employs this principle in conjunction with the introduction of a 180xc2x0 phase shift into one of the two radar signals retransmitted from spaced locations on the carrying craft to cause wave front distortion at the location of the victim radar. It is essential that the overall phase shift introduced into the retransmitted pulse be extremely accurate in that a variation of more than xc2x12-6xc2x0 of shift causes a beacon effect which easily identifies the location of the carrying craft. The practical application of this technique within the confines of present day technology has been somewhat difficult because of the necessity of maintaining the narrow phase shift tolerance between the enemy radar pulse and retransmitted countermeasure radar pulses.
One prior art countermeasure system utilizing the VanAtta array principle is known as the xe2x80x9cswitching cross-eyexe2x80x9d and involves the use of a first microwave antenna located at the extremity of one portion of the carrying craft, such as the tip of one wing, while a second and identical antenna is positioned at another extremity of the aircraft, such as the tip of the other wing. A waveguide leads from each antenna to a single time multiplexing circuit disposed about midway on the vehicle between the two antennas. The multiplexing circuits comprises a pair of switches which must be capable of operating at switching speeds in the order of two to three times the victim radar bandwidth to sample the incoming radar signals. The circuit then stores each time slice in a single delay line to mitigate the effect of reflections that occur when the high power radiated signals are applied to common antennas. After the time multiplex signal emerges from the delay line, it is supplied to a single amplifier and then to the second switch for distribution to the appropriate antenna port. Although the use of only one amplifier and delay line in a common channel causes a balancing of the phase distortion in the two retransmitted signals, the high switching speed required of the switches located at either end of the common channel is beyond the capability of present day RF switches.
In a commonly owned application, Ser. No. 519,465, by S. Brody, and G. Bock on Aug. 27, 1971 entitled SINGLE AMPLIFIER COUNTERMEASURE (U) there is disclosed a track radar countermeasure utilizing three channels in which a single amplifier is employed in the center channel to amplify the received energy for redirection to the enemy radar. The outside two channels store phase information and are used to recreate the correct phase.
The aforementioned copending application is an improvement over the invention disclosed in another commonly-owned copending application, Ser. No. 334,172, by Richard C. DiDomizio and Lester H. Kosowsky filed on Feb. 12, 1973 entitled AUTOPHASED ANGLE DECEPTION COUNTERMEASURE (U) in which the differences between the phases of signals received at three antennas from the enemy radar is utilized to generate a phase adjustment to the central antennas so that the redirected waves have a proper phase relationship even though the enemy radar is not located on a normal to the alignment of the antennas. Both of the hereabove cited patent applications, and the references cited therein, should be consulted for the purpose of putting the present invention in perspective.
The principal object of the present invention is to provide an improved track radar countermeasure system in which two common channels are utilized to reduce phase distortion inherent in a channel containing amplifier, delay line, etc. for processing radar frequency signals.
According to the present invention, a switching cross-eye countermeasure system utilizes two common channels between two spaced apart antennas so that any phase distortion introduced by the waveguides, amplifiers, etc. will affect the retransmitted pulses equally, rather than exceeding the phase tolerance limits acceptable for the introduction of distortion into spherical wave front redirected toward the enemy radar.
According to the present invention, a switching cross-eye countermeasure system is disclosed in which two common channels are provided for initially storing an enemy radar signal. Both ends of each common channel are controlled by an RF switching element to create a single common path through which a received pulse propagates from one port to the other port, and vice versa. The received pulse is delayed in the delay line of one common channel for a period at least as long as the pulse width of the received radar pulse and then the switches are transitioned to allow the pulse through to the second common channel.
Other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of a preferred embodiment thereof, as illustrated in the accompanying drawing.